This invention relates to pulsed plasma processing systems used for ion implantation of workpieces and, more particularly, to methods and apparatus for operating such systems at low implant voltages.
Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. In a conventional ion implantation system, a desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Exacting requirements are placed on semiconductor fabrication processes involving ion implantation with respect to the cumulative ion dose implanted into the wafer, implant depth, dose uniformity across the wafer surface, surface damage and undesirable contamination. The implanted dose and depth determine the electrical activity of the implanted region, while dose uniformity is required to ensure that all devices on the semiconductor wafer have operating characteristics within specified limits. Excessive surface damage, particularly chemical etch, or contamination of the surface can destroy previously fabricated structures on the wafer.
In some applications, it is necessary to form shallow junctions in a semiconductor wafer, where the impurity material is confined to a region near the surface of the wafer. In these applications, the high energy acceleration and the related beam forming hardware of conventional ion implanters are unnecessary. Accordingly, it has been proposed to use Plasma Doping (PLAD) systems for forming shallow junctions in semiconductor wafers.
In a PLAD system, a semiconductor wafer is placed on a conductive platen located in a chamber, and the platen functions as a cathode. An ionizable gas containing the desired dopant material is introduced into the chamber, and a high voltage pulse is applied between the platen and an anode (or the chamber walls), causing the formation of a plasma having a plasma sheath in the vicinity of the wafer. The applied voltage causes ions in the plasma to cross the plasma sheath and to be implanted into the wafer. The depth of implantation is related to the voltage applied between the wafer and the anode. A plasma doping system is described in U.S. Pat. No. 5,354,381 issued Oct. 11, 1994 to Sheng.
In the PLAD system described above, the high voltage pulse generates the plasma and accelerates positive ions from the plasma toward the wafer. In other types of plasma implantation systems, known as Plasma-Source Ion Implantation, PSII, systems, a separate plasma source is used to provide a continuous plasma. (These implantation systems are also known by several other acronyms, the most common being Plasma-Immersion Ion implantation, PIII.) In such systems, the platen and the wafer are immersed in this continuous plasma and at intervals, a high voltage pulse is applied between the platen and the anode, causing positive ions in the plasma to be accelerated toward the wafer. Such a system is described in U.S. Pat. No. 4,764,394, issued Aug. 16, 1988 to Conrad.
An advantage of a PLAD system over a PSII system is that the plasma is on only when the target object is being implanted. This results in a reduction of chemically active species that are produced by the continuous plasma of the PSII system and hence a reduction in chemical damage to the wafer surface. In addition, the continuous plasma can also cause high levels of implanted contaminants and high levels of particulate formation. The PLAD system improves on the PSII system by turning the plasma off except when the target object is biased to implant ions. This reduces the level of contaminants, particulates and surface etching damage.
PLAD systems have a minimum breakdown voltage Vbd at which the plasma ignites and ions can be implanted. This breakdown voltage Vbd is defined by the physical characteristics of the system, including the cathode surface material, the type of gas present in the system, the pressure P of the gas in the system and the distance d from the cathode to the anode. For a given surface material and gas type, the breakdown voltage curve Vbd is a function of Pxc3x97d and is known as the Paschen curve. The process is well described in plasma physics texts. Typically, the minimum value for the breakdown voltage Vbd is near Pd≈500 millitorr-cm. For BF3, a common feed gas used for PLAD of Si, the minimum breakdown voltage Vbd≈xe2x88x92530 V. Other dopant feed-gas/substrate combinations will have similar minimum breakdown voltages Vbd. The implant energy of the ions in the plasma is directly proportional to the cathode to anode voltage in prior art PLAD systems.
In PLAD systems, the ion current to the cathode is a function of the applied voltage, gas pressure, and surface conditions. For voltages near Vbd, the current is low. As the voltage or the pressure is increased, the current increases. In order to increase current and thereby reduce implant times, it is desirable to operate at higher pressures and voltages above Vbd. Local surface conditions, surface temperature, material, material structure (crystal vs. amorphous), etc., also play a role in the local ion current.
It is envisioned that future generations of integrated circuits will require ultra-shallow junctions. Conventional PLAD systems, however, have implant energies, due to the breakdown voltage Vbd, which are too high for the production of some ultra-shallow junctions.
Thus, there is a need for a PLAD system capable of implanting dopant materials at low energies, i.e., voltages, and at high currents to permit formation of ultra-shallow junctions with short implant times.
According to a first aspect of the invention, a method of implanting ions in a target within a pulsed plasma doping system is provided. The method comprises providing an ignition voltage pulse Vplas to an ionizable gas to create the ions for implantation; and providing an implantation voltage pulse Vimp to the target to implant ions into the target, wherein the implantation voltage pulse Vimp and the ignition voltage pulse Vplas overlay in time at least partially and have one or more different parameters.
Preferably, the ignition voltage pulse Vplas is coupled to a plasma ignition cathode; and the implantation voltage pulse Vimp is coupled to an implantation cathode on which the target is mounted. The ignition voltage pulse Vplas and the implantation voltage pulse Vimp may be provided by, respectively, first and second high voltage pulse sources.
The implantation voltage pulse Vimp may be a function of the ignition voltage pulse Vplas. This is accomplished, in one embodiment, by coupling a voltage divider network to the ignition voltage pulse Vplas; and generating the implantation voltage pulse Vimp from the voltage divider network. The implantation voltage pulse Vimp amplitude may be less than the ignition voltage pulse Vplas amplitude.
The implantation voltage pulse Vimp may be substantially concurrent with the ignition voltage pulse Vplas. Alternatively, the implantation voltage pulse Vimp may start after a start of the ignition voltage pulse Vplas and end prior to an end of the ignition voltage pulse Vplas. Still further, the implantation voltage pulse Vimp, may begin before and end after the ignition voltage pulse Vplas. The implantation voltage pulse Vimp may occur after the ignition voltage pulse Vplas has ended.
According to another aspect of the invention, a pulsed plasma doping system for implanting ions in a target is provided. The system comprises means for providing an ignition voltage pulse Vplas to an ionizable gas to create the ions for implantation; means for providing an implantation voltage pulse Vimp to the target to implant ions into the target; and
means for controlling a timed relationship between the implantation voltage pulse Vimp and the ignition voltage pulse Vplas to overlay in time at least partially and have one or more different parameters.
In yet another aspect of the present invention, a pulsed plasma doping apparatus for implanting ions in a target is provided. The apparatus comprises a first high voltage pulse source to provide an ignition voltage pulse Vplas to an ionizable gas to create the ions for implantation; and a first device to provide an implantation voltage pulse Vimp to the target to implant ions into the target, wherein the implantation voltage pulse Vimp and ignition voltage pulse Vplas overlay in time at least partially and have one or more different parameters.
In still another aspect of the present invention, a pulsed plasma doping system for implanting ions in a target is provided. The system comprises a vacuum chamber to contain an ionizable gas; an anode coupled to a reference voltage, the anode disposed within the vacuum chamber; a plasma source cathode disposed within the vacuum chamber; a first high voltage pulse source to provide an ignition voltage pulse Vplas to the plasma source cathode; an implantation cathode to support the target; and an implantation voltage pulse source to supply an implantation voltage pulse Vimp to the implantation cathode. The implantation voltage pulse Vimp and the ignition voltage pulse Vplas overlay in time at least partially and have one or more different parameters.